This invention relates to the prevention of fouling in
which is in contact with a gaseous or liquid hydrocarbon stream.
In cracking operations, such as in the pyrolytic cracking of ethane, propane, and naphtha to form olefins, oxygenated compounds, including carbonyl compounds, are formed. The amount of carbonyl compounds, such as aldehydes and ketones, formed in such operations can vary widely, but is typically about 1-100 ppm in the gas stream with concentrations as high as 1000 ppm occasionally being encountered because of the utilization of various feedstocks and cracking temperatures. When the gas stream is passed through a basic wash (pH &gt;7) to remove acidic components such as hydrogen sulfide and carbon dioxide, oxygen containing compounds, such as carbonyl compounds, are also removed. These oxygen containing compounds, particularly acetaldehyde, will undergo polymerization in the presence of the basic wash or scrubbing conditions. In the wash tower, the resulting polymer settles on the trays leading to fouling and eventual plugging of the trays. Eventually the unit must be shut down for cleaning-- obviously a costly operation. The basic wash systems, where treatment is required to inhibit such polymer-based fouling, include amine acid gas scrubbers (e.g., MEA, DEA, isopropyl amine, butyl amine, etc.) and caustic wash systems.
Polymer based fouling is inhibited in basic (pH 7) wash systems of the type adapted to remove impurities from liquid or gas phase hydrocarbon mediums by adding to the wash system a carbohydrazide compound. As used herein, "carbohydrazide compound" means a compound whose structure is shown by the formula H.sub.2 NNHCONHNH.sub.2. Carbohydrazide is also known as carbazide and sometimes referred to as carbodihydrazide.
The carbohydrazide treatment is well suited for inhibition of polymer based deposits formed during the caustic scrubbing of gas phase olefinic hydrocarbons resulting from pyrolytic cracking processes. Heretofore, when such gas phase olefinic hydrocarbon streams were subjected to caustic wash systems, the carbonyl compounds therein, including ketone and aldehyde contaminants, tended to undergo Aldol condensation, forming insoluble polymer molecules. The polymers deposited on system equipment and, for instance, tended to plug the trays in the caustic wash tower.
The inventive method is particularly appropriate for the basic washing process which follows the pyrolytic cracking of such hydrocarbons as ethane, propane, butane, naphtha and mixtures thereof to produce the corresponding gaseous ethylene, propylene, butadiene and the like. These product streams contain the carbonyl and other contaminants.
Generally, the basic washing entails contacting the gaseous olefins with an aqueous basic solution in a wash tower to remove hydrogen sulfide, carbon dioxide and oxygenated compounds therefrom. The conditions in the wash tower are conducive for condensation reactions of any aldehydes (e.g., acetaldehyde) and/or ketones contained therein.
The present method entails assuring that the basic wash takes place in the presence of the carbohydrazide compounds. For example, carbohydrazide has been found to be an effective anti-polymerant for ethylene unit caustic towers and amine units used to scrub gases after the furnace but prior to the recovery section. During ethylene production, a small amount of partial oxidation products are formed. The major component of these partial oxidation products is acetaldehyde. The effluent from the ethylene furnace, containing acetaldehyde and other oxidation products, is washed in a caustic tower. This process removes acid gases such as carbon dioxide and hydrogen sulfide.
ln some ethylene production units, an amine unit is used in front of the caustic tower to remove most of the acid gases. On contact with a caustic or amine solution, acetaldehyde and other aldehydes or ketones undergo a base catalyzed Aldol condensation. The result of these numerous Aldol reactions is a water insoluble polymer.
At some ethylene manufacturing facilities, a vinyl acetate plant is also present. Ethylene is used in the vinyl acetate production process. Unreacted ethylene is recovered by distillation and sent back through the ethylene unit fractionization train. Vinyl acetate can be entrained with the unreacted ethylene and enter the fractionization train. When vinyl acetate reaches the caustic tower it is hydrolyzed to produce a salt of acetic acid and vinyl alcohol. Vinyl alcohol tautomerizes to acetaldehyde, a source of the fouling polymer.
In U.S. Pat. No. 4,673,489, hydroxylamine and its hydrochloride and hydrogen sulfate salts have been used to inhibit polymer formation caused by condensation reactions of aldehydes contained in caustic scrubber units. However, despite the success of such treatments, these compounds are expensive and must be overfed to the caustic scrubber units in light of their apparent selectivity toward the ketone contaminant.
Similarly, in U.S. Pat. No. 4,952,301, ethylenediamines and water soluble salt forms thereof have been used to inhibit carbonyl based fouling, particularly aldehyde fouling, that often occurs during caustic scrubbing of liquid or gas phase hydrocarbon streams. However, the ethylenediamine compounds are expensive, and the method described herein using the carbohydrazide compounds provides a more economical treatment approach.